Production of propylene oxide



Nov. 2l, 1950 G. A. cooK PRODUCTION oF PROPYLENE oxIDE Filed Feb. 14, 1947 INVENTOR GERHARD A. COOK ATTO Patented Nov. 2l, 1950 PRODUCTION F PROPYLENE OXIDE Gerhard A. Cook, Snyder, N. Y., assignor to The Linde Air Products Company, a corporation of Ohio Application February 14, 1941, sei-nn No. 72am 9 claim. (ci. zoo-sass) l This invention relates to the production of propylene oxide and more particularly to an improved process for synthesizing this compound by direct oxidation of propane, propylene or mixtures of these hydrocarbons.

An economical method for producing propylene oxide by direct oxidation of three carbon atom hydrocarbons has been long sought but to my knowledge no commercially successful process of this type has heretofore been developed. The chief obstacle lying in the path of this achievement has been the nature of the reaction. It comprises a partial oxidation of hydrocarbon gas, which reaction,-when once initiated, has a strong tendency to proceed to complete oxidation, while at the same time the effect of the reaction promotes isomerization to compounds other than the oxide. For this reason, when propane or pro-` pylene hydrocarbons previously have been oxidized directly, generally such compounds as aldehydes, alcohols, ketones, acids, carbon dioxide and Iwater have been produced, with little or no yield of propylene oxide. While several processes have been advanced asserting production of propylene oxide by direct oxidation, none of these reveal or claim yields in quantities which manifest commercial feasibility of such processes. The use of catalysts, such as silver and gold, which have served well to provide means for the commercial production of ethylene oxide by direct oxidation of ethylene, have not been eilective in the direct oxidation of propylene hydrocarbons.

The lack of a practical direct oxidation process is evidenced quite emphatically by the fact that the only process known to be practised today on a commercial scale is an indirect one, whereby propylene chlorhydrin is first prepared from propylene, chlorine and water, from which propylene oxide is obtained by subsequent alkaline treatment of the chlorhydrin. By this procedure, high operating costs are involved, and the equipment for handling the highly corrosive and voluminous quantities of intermediate and final products must necessarily be of large size and expensive. Thus the advantages in a simple inexpensive direct oxidation process are apparent, and, with continually increasing demand for propylene oxide as the raw material for synthesizing new products, the value of these improvements in the production of this chemical is further emphasized.

It is an object of this invention to provide a new and improved process for producing propylene oxide from three carbon atom hydrocarbons by direct oxidation. A further object is to disclose a process for producing propylene oxide by direct partial oxidation of propane and mixtures of propane and propylene, as well as from propylene alone, which processemploys simple and inexpensive equipment and is. commercially economical in operation.

The invention is based essentially on the discovery that this direct oxidation reaction can best be conducted in a vessel adapted to provide a reaction zone having an appreciable amount of gas contacting surface, or an area of such surface relative to the total volume of free reaction space, which is substantially in excess of what has been considered, heretofore. to retard more or less completely the intended reaction. With control of this factor, coupled with a reaction pressure above atmospheric, and preferably increasing with increase in the `ratio of lsurface area to volume of free space, there have been produced propylene oxide yields, as derived from both propane and propylenefwhich will exceed those previously obtained in the art of direct oxidation.

Any vessel which is adapted to provide a reaction zone having a ratio in square centimeters of contacting surface area to cubic centimeters of free space volume exceeding 1.6 is suitable for serving as the apparatus for practising the invention. Such a reaction vessel may be one in the shape of a cylinder having a relatively small diameter, one having the form of a rectangularv parallelepiped with one or two of its three dimensions-substantially smaller than its others or other, one which is packed with a material v to increaseits gas contacting surface while decreasing its interior free space, or one containing any of the above features of shape and packing.

In operation, a pressure in excess of atmospheric pressure should be maintained in the reaction zone. It has been found that'maximum yields of propylene oxide are obtained when the sum of the partial pressures of the reactants (hydrocarbon gas and oxygen) bears a definite relationship to the ratio of surface to free space within the reaction vessel. In general, the higher the ratio of surface to free space, the higher must be the total partial pressure of the reactants. While almost any ratio of surface to free space in the reaction zone above 1.6 square centimeters per cubic centimeter is contemplated as satisfactory, it has been found that reaction vessels adapted to provide a ratio on the order of 5 or greater, give higher yields. The range of pressure which can be used to produce satisfactory yields varies from a pressure above atmospheric to any pressure which may be conveniently handled provided that. at pressures above about 100 pounds per square inch absolute, the proportion of oxygen in the reaction mixture is kept relatively low. However, with operation adaptable to economic 'yields of propylene oxide at sufficiently low operating pressures to provide a wise balance between the cost of production and cost of equipment, it has been found that pressures between 20 and 200 pounds per square inch absolute are advisable. For example, it has been observed that in a reaction zone having a ratio of surface to free space of about 8 square centimeters per cubic centimeter, maximum yields of propylene oxide will be obtained if the sum of the partial pressures of the reactants is maintained between 20 to 60 pounds per square inch.

The accompanying drawing illustrates one form of apparatus which is suitable for the production of propylene oxide by the process of this invention.

The hydrocarbon gas, which is stored in a suitable gas holder, is fed into the apparatus through valve l at a predetermined measured flow. The gas is compressed by circulating pump 2 which is designed and has controls for regulating theV pressure throughout the apparatus. Gas leaving pump 2 is mixed with an oxygen containing gas which is fed under pressure into the apparatus at a measured rate of flow through valve 3. The resulting gas mixture passes into reactor 4 where the hydrocarbon gases are oxidized. Reactor 4 is essentially in the shape of a heat exchanger consisting of a bundle of steel tubes enclosed in a steel jacket which in turn is surrounded by a heating coil 5. The steel tubes are coated internally with a thin layer of ceramic cement to render them inert to the reaction mixture and are lled with non-porous porcelain Raschig rings to provide a desired ratio of surface to free space by increasing the surface and reducing the free space of the tubes. Coil 5, which surrounds reactor 4, may be an electrical resistance heater or a uid heater employing a high boiling liquid, such as a eutectic mixture of phenoxybenzcne and phenylbenzene. It serves for heating reactor 4 to the temperature necessary to start and control the reaction. A stirred salt bath of 50 per cent potassium nitrate and 50 per cent sodium nitrite is maintained in the shell side of reactor 4 for serving as a heat transfer and heat absorption medium. When reactor 4 is being heated by coil 5 preparatory to operation, the salt bath transfers its heat uniformly for raising the temperature of the steel tubes to reaction temperature. After the reaction has begun the salt bath functions in addition to absorb part of the heat of the exothermic reaction away from the inside of the tubes. It acts as a heat ballast to prevent any sudden change in temperature within reactor 4. Temperature indicating devices y immersed in the salt bath provide a means for determining and controlling the temperature of the salt.

The hot reaction products, along with the unreacted gaseous mixture, pass through heat exchanger 6, where they are cooled to a temperature at which all reaction ceases, and they then enter scrubber l. Scrubber 'l is a steel shell packed with stoneware Raschig rings into which water, buffered with 0.2 per cent by weight of sodium bicarbonate (to hinder hydrolysis of propylene oxide to propylene glycol), is fed by pump 8 at the top. The water solution percoter soluble constituents ot the reaction product entering at the base of the scrubber. The hot mixture of water and water soluble constituents is drawn off at the bottom of the scrubber by pump 9 where the mixture is divided, one portion being withdrawn from the system through valve l0 for separation of the propylene oxide from the water and other water soluble products and the remaining portion being passed through cooler Il where its temperature is reduced to 15 to 25 C. to retard hydrolysis of propylene oxide to its glycol. The cooled mixture is recycled back into scrubber l at about its center for concentrating it with additional water soluble constituents and for cooling the scrubber liquid. The mixture of water insoluble gases passing through scrubber 1 may be entirely withdrawn from the system through valve I2 or part may be recycled with fresh hydrocarbon gas lthrough valve i3. 1f the gases are recycled, usually from about 15 to 30 per cent of the gases leaving the scrubber are purged through valve i2. While propylene oxide may be produced in good yield in a non-recycling system, greater eiliciency results if part of the gas leaving the scrubber serves as a portion of the reaction mixture. The desirability of recycling will be evident when it is consideredethat the gas leaving the scrubber has a .hydrocarbon mole contentof about 10 to 90 per cent. YIt is inadvisable to lower this hydrocarbon content by decreasing the percentage of three carbon atom hydrocarbons forming part of the reaction mixture because yields will suii'er if this is attempted. For optimum yields it is necessary to maintain an excess of hydrocarbons in the reaction mixture because substantially all of the oxygen reacts and it is impractical to attempt to lessen this degree of consumption.

The composition of the feed gas is of major importance in determining optimum operating conditions. Its reactants are three carbon atom hydrocarbons and oxygen. The composition of the hydrocarbons may be pure propane, pure propylene or mixtures of both. While the percent conversion of hydrocarbons to propylene oxide, as well as the space-time yields (weight of product produced in a unit of time Der volume of reaction space) of propylene oxide are higher if the hydrocarbons are largely propylene rather than propane, nevertheless the abundant nature and low cost of propane may more than oiset this disadvantage. However, since the chief source of propylene is from propane, an unrened product comprising a mixture of propane and propylene which is yielded in the production of propylene manifests certain economic raw material advantages. This mixture comprises from about to per cent propylene and about 25 to 15 per cent propane, and its use combines excellent yield and eiliciency with relatively low raw material cost. Regardless of whether pure propane, pure propylene or mixtures of both are employed, it is advisable to select hydrocarbon gases which do not contain high concentrations of oxidizable constituents other than three carbon atom hydrocarbons. Hydrocarbons, such as ethylene and butylene, compete with the desired hydrocarbons for the oxygen and when oxidized give off large quantities of heat. Their simultaneous reaction causes decomposition of some of the gases to carbon and promotes isomerization of the propylene oxide to less desirable compounds. However, the

`feed gas may contain small amounts of these hydrocarbons and yet give yields of propylene oxide lates down through the scrubber to dissolve wa- 75 inreduced quantities.

The other active ingredient of the reaction 'mixture is ongen. It may be added to the h ydrocarbon gas in the form of pure oxygen or an oxygen containing gas. such as air. From an efficiency standpoint air may be preferred to pure oxygen. However, from an economic standpoint the higher efficiency obtainedl with air is partially or wholly balanced by higher cost of operation. The large volume of nitrogen in air which must be processed along with the other gases increases the cost of pumping, the cost of scrubbing 'all of the propylene oxide from large volumes of a dilute concentration of this material, and the cost of recovering unreacted hydrocarbons from the purge gas.

In order to direct the oxidation of the reaction mixture toward the production of propylene oxide rather than toward more completely oxidized products, it is advisable to maintain the molar concentration of its hydrocarbons greater than that of its oxygen content. Maximum conversions to propylene oxide are obtained when substantially all the oxygen is consumed during each pass through the reactor, and when the molar ratio of three carbon atom hydrocarbon content to oxygen content of the reaction mixture is within the range of2 to 20. Molar ratios in this range are satisfactory for both single pass and recycling operation. However, molar ratios in the range of 3 to 8 give best eiciencic in recycling operations.

'I'he temperature at which suitable reaction takes place varies, depending on the composition of the reaction mixture and the pressure at which the reactor is maintained. For reaction mixtures having molar ratios in the recommended range, the minimum temperature of reaction generally decreases as the pressure of the reactor is increased, within a limited temperature range. For example, a bath temperature as low as 275 C. is sufficient to support reaction at a reactor pressure of about 95 pounds per square inch absolute, and on the other hand, gas temperature may rise to 700 C. under some conditions, but it is preferred that the gas temper` ature be kept below about 550 C. Usually the bath temperature at which the reaction is initiated is higher than that required to support the reaction thereafter. Bath temperatures within the range of 325 C. to 425 C. have been found most satisfactory at operating pressures below 200 pounds per square inch absolute for obtaining maximum efficiencies and yields. Whenever an optimum temperature is determined, operation should be directed to prevent the bath from rising substantially above this temperature, since prolonged exposure of propylene oxide to higher temperatures leads to its beingl isomerized, cracked, or further oxidized.

All of the temperatures referred to in the specification and claims are the temperatures of the bath surrounding the reactor. Such temperatures are given because they can be readily and directly controlled and can be ascertained more easily and with greater accuracy than those of the reactor.

To contribute to high efliciency and good yields, localized high temperatures should be prevented and excess heat of reaction should be quickly removed. One of the simplest means for accomplishing this is by the apparatus illustrated and previously described. The heat of reaction is uniformly distributed and excessive heat is quickly removed therefrom by the large external surface of the reactor tubes along with the assistance of the molten salt heat transfer medium or other high boiling liquid such as a eutectic mixture of phenoxybenzene' and phenyl benzene. Other means having features which lend themselves to achieve this result are the use of a rsactor with a bed of' packing containing a cooling coil or cooling pipes installed therein through which a heat transfer liquid is circulated to maintain a predetermined temperature within the reactor; by dilution of the reaction mixture with an inert gas such as nitrogen, steam and the like, or with a large excess of hydrocarbons which function to spread thereaction through a.` larger mass of gas to absorb the excess heat of reaction; by providing a moving bed of packing within a reactor, which packing absorbs and carries on' excess heat from the zone of reaction; or by a regenerator system in which the direction of flow of the reaction mixture through the reactor is periodically reversed to cause the heat to be first absorbed in one zone of the reactor which on flow reversal provides preheat for the reaction mixture while a cool zone is made availr able for subsequent absorption of heat. It should be noted that when substances other than oxygen and three carbon atom hydrocarbons are present in the reaction mixture, such as in the case in which air or diluents are used, the effective pressure may be defined as the sum of the partial pressures of the oxygen and hydrocarbons. When air is used as a means for providing oxygen or when diluents, such as nitrogen and steam are added for assisting in obtaining good heat distribution, the actual operating pressure must necessarily be in excess of the effective pressures of the reactants.

The length of time for heating the gas mixture up to reaction temperature may be varied within a range of 0.l to 50 seconds without greatly changing efficiency or yield. It is believed that the reaction itself takes place in a relatively short time and space and that it is diicult if not impossible to lengthen the reaction time. The use of a long tube reactor operated with a relatively long heating zone to increase its heating time does not significantly change the efliciency and 1yield from that obtained by a shorter heating ime.

The material which forms the surface of the reaction vessel and of which the packing is composed should be inert to the reactants. Almost any kind of ceramic material appears to haveI these properties. Good results have also been obtained with stainless steel and aluminum. The use of both copper and silver should be avoided because their oxides catalyze the formation of carbon dioxide. For operation of the reactor at lower pressures (20 to 65 pounds per square inch absolute), porous surface material should not be selected because the exceedingly large surface presented by porous materials would require a correspondingly high pressure of reaction to effect good eiiiciency and yields.

' The shape of the reactor packing material, if one is used, 'appears to have little bearing on operating conditions. Rings, and broken shapes are both suitable. The shape of the packing selected, therefore, will depend on the type of reactor used and the ratio of surface to free space desired.

The yield of propylene oxide may be recovered in maximum amounts if it is separated from the reaction products as soon as possible after it is produced. Propylene oxide will isomerize or del compose if left at or near the oxidation reaction temperature for any considerable length of time and in aqueous solution it. tends to hydrolyze. Buffering to pH 7 to 8 greatly retards this hydrolysis. If it is -not practical to separate the propylene oxide from the other reaction products within a day or two of the time it is produced, cooling or refrigerating the entire reaction product will retard formation oi' propylene glycol.

Example 1 An apparatus similar to the type illustrated and described was used for producing propylene oxide in a recycling system. The reactor was constructed of 14 eight foot long steel tubes, having inside diameters of 11A; inches and being coated on the tube inner surface with a refractory cement. 'I'he tubes were welded into tube sheets supported by a shell and were filled with inch non-porous Raschig rings to provide a ratio of surface to free space for the reaction chamber of approximately 7.9 square centimeters per cubic centimeter. The shell was filled with molten salt to a depth of three feet and means were provided for stirring the salt. The scrubber was built of two 4 foot sections of 8 inch pipe with a seven gallon reservoir attached at its bottom. All of the scrubber except this reservoir a was packed with 41 inch stoneware Raschig rings to provide an effective absorption system.

In operating the equipment, gaseoi reaction mixture, composed of fresh propane, pure oxygen and recycle gas from the scrubber and having a molar ratio of hydrocarbon to oxygen of 3.7, was passed through the reactor at a rate oi 630 cubic feet per hour. The reactor was maintained at a pressure of 57 pounds per square inch absolute and the salt bath surrounding its bottom section was held at 400 C., once the reaction had been initiated. Under these conditions, it was calculated that each molecule of the reaction mixture remained in the heated zone of the reactor for approximately two seconds. After Example 2 The same equipment as described in Example 1 served for this operation. A reaction mixture having a molar ratio of three carbon atom hydrocarbons to oxygen of 3, which was prepared from fresh propane, air and recycle gas, was pumped through the reactor at a rate of 910 cubic feet per hour. The reactor was held at 165 pounds per square inch absolute pressure, the salt bath was maintained at 365 C. and the total heating time per molecule of reaction `mixture under these conditions was calculated to be approximately 4.1 seconds. Of the 900 cubic feet per hour flow of gas leaving the scrubber, 24 per cent was purged and the remainder was recycled as part of the reaction mixture. Propylene oxide was produced at a rate of 5.8 milligrams per liter of reaction mixture, which eiiected a space-time yield for the run of 1.04 pounds of propylene oxide per cubic foot of reaction space per hour of operation.

Example 3 Into the same equipment used for the previous examples, a reaction mixture o! fresh hydrocarbons, comprising about 85 per cent propylene and 15 per cent propane, pure oxygen and recycle gas, proportioned to provide a molar ratio of three carbon atom hydrocarbons to oxygen of 4.9 was fed at a rate of 600 cubic feet per hour. The pressure o! the reactor was pounds per square inch absolute, and its salt bath was held at 385 C. Gas was discharged from the scrubber at the rate of 534 cubic feet per hour of which 16.5 per cent was purged and the remainder recycled in the system as part oi the reaction mixture. A yield of 34.5 milligrams of propylene oxide per liter or reaction mixture was obtained, which provided a space-time yield of 4.08 pounds of propylene oxide per cubic foot of reaction space per hour oi' production.

Example 4 In the same equipment; propylene oxide was produced from a reaction mixture having a molar ratio of 4.6 which was prepared from fresh hydrocarbons comprising about per cent propylene and 15 per cent propane, air and recycle gas. The reactor was maintained at pounds per square inch absolute pressure with its salt bath held at 365 C., and its heating time was 'about 3.9 seconds for each molecule of reaction mixture when fed therein at the rate of 945 cubic feet per hour. Gas issued from the scrubber at the rate of 916 cubic feet per hour of which 24 per cent was purged and the remainder served as recycle gas for the reaction mixture. Propylene oxide was produced at the rate of 10.3 milligrams per liter of reaction mixture, giving a space-time yield of 1.90 pounds of propylene oxide per cubic foot of reaction space per hour of operation.

Example 5 A non-recyling reactor was constructed of a1 inch seamless steel tube. welded concentrically through two discs to a 6*/2 foot long pipe which was 3% inches in diameter. The 1 inch pipe was packed with inch porcelain Raschig rings to provide a reaction space having a ratio of surface to free space of about 8. In the annular space between the 1 inch pipe and 31/2 inch pipe, e. stirred molten salt bath was provided and maintained at about 350 C. A mixture of propylene and oxygen in a molar ratio of 5.2 was passed at a pressure oi 45 pounds per square inch absolute through the heated reactor. The ilow was regulated to subject each molecule of gas mixture to 16 Seconds of heating. A yield of 49 milligrams of propylene oxide was obtained per liter of reaction mixture (measured at 0 C. and 1 atmosphere absolute pressure). A space-time yield of 0.74 pound of propylene oxide per cubic foot of reaction space per hour of production was achieved.

These examples illustrate only limited phases 7 of the invention. It should be understood that propane. 'I'he ratio of surface to free space may be varied substantially from the range illustrated snow along with corresponding variations in the presrecovered from the scrubber liquid is of no important concern oi' the present invention. It may be separated and purified by either physical or chemical means.v Good yields have resulted by ilashing oil the propylene oxide under a partial vacuum, and then purifying it by fractional distillation.

The type of apparatus used for the propylene oxide production may be of many designs without seriously decreasing yields and yet being operable within the scope of the process of the invention. While that apparatus illustrated and described most simply performs the process, other types of apparatus have been disclosed which may be used. Instead of reacting the three carbon atom hydrocarbons with oxygen in a solid packed reactor, reaction may be conducted in an inert liquid medium whose temperature is controlled, such as bubbling the reactants through a thermally heated and controlled salt bath. The pressure at which reaction occurs may be changed to an optimum by varying the depth at which the reactants are issued into the salt bath.

In practising the process of this invention, space-time yields of propylene oxide have been signiiicant. Yields of a magnitude up to forty times that indicated by any other reported direct oxidation process have been obtained. In addition, the simplicity and inexpensive nature of the equipment for carrying out this process serve favorably in comparing its economy with any other process for producing propylene oxide. Moreover, the fact that propane may be used as a raw material as well as propylene, introduces a great economy in the cost of raw material. Propane is available in much more abundant quantities and its cost is considerably less than that of propylene. No other direct process is known which produces propylene oxide from propane.

It is to be understood that the invention is not limited to the speciiic embodiments disclosed herein except as defined by the following claims.

I claim:

l. A process for producing propylene oxide by direct oxidation which comprises reacting, at a temperature within the range of about 275 C. to about 700 C., hydrocarbon gas selected from the group consisting of propane and propylene with a gas containing molecular oxygen in amount suflicient to give in the total gas mixture a molar ratio of hydrocarbon to oxygen of at least 2 and not more than 20, while confining the gases in a reaction zone having a'ratio of area of gas contacting surface to volume of free space of at least 5 square centimeters per cubic centimeter, and maintaining a pressure above atmospheric on the reacting gases.

2.'A process for producing propylene oxide by direct oxidation which comprises reacting, at a temperature within the range of about 275 C. to about 700 C., hydrocarbon gas selected from the group consisting oi' propane and propylene with a gas containing molecular oxygen in amount sufilcient to give in the total gas mixture a molar ratio of hydrocarbon to oxygen of at least 2 and not more than 20, while conning the gases in a reaction zone having a ratio of area of gas contacting surface to volume of free space of at least 5 square centimeters per cubic centimeter, and maintaining the sum of the partial pressures oi hydrocarbon and oxygen inthe range of about` 20 to 200 pounds per isquare inch.

3. A process for producing propylene oxide by direct oxidation which comprises reacting hydrocarbon gas selected from the group consisting ot propane and propylene with a molecular oxygen containing gas admixed to give a molar ratio of hydrocarbon to oxygen content within the range of about 2 to 20, said reaction being conducted at a temperature within the range of about 275 C. to 500 "C. in a conned reaction zone having a ratio of area of gas contacting surface to volume of free space of at least 5 square centimeters per cubic centimeter, while maintaining the sum of the partial pressures of hydrocarbon and oxygen in the range oi' about 20 to 200 pounds per square inch.

4. A process for producing propylene oxide by direct oxidation which comprises reacting hydrocarbon gas selected from the group consisting of propane and propylene with a molecular oxygen containing gas admixed to give a molar ratio of hydrocarbon to oxygen content within the range of about 3 to 15, said reaction being conducted at a temperature within the range oi' about 275 C. to about 450 C. in a confined reaction zone consisting essentially of at least one tube packed with an inert material having a ratio of area of gas-contacting surface to volume of free space of at least 5 square centimeters per cubic centimeter while maintaining the sum of the partial pressures of hydrocarbon and oxygen in the range of about 20 to 200 pounds per square inch.

5. A process for producing propylene oxide by direct oxidation which comprises reacting hydrocarbon gas selected from the group consisting of propane and propylene with oxygen admixed to give a molar ratio of hydrocarbon to oxygen Within the range of 3.5 to 5.5, said reaction being conducted at a temperature within the range of about 350 C. to about 450 C. in a conned reaction zone consisting essentially of at least one tube packed with an inert material having a ratio of area of gas contacting surface to volume of free space in the range of about 5 to 15 square centimeters per cubic centimeter, while maintaining a pressure in the range of to pounds per square inch absolute on the reacting gases.

6. A process for producing .propylene oxide by direct oxidation which comprises reacting hydrocarbon gas selected from the group consisting of propane and propylene with air admixed to give a molar ratio of hydrocarbon to oxygen content within the range of about 3 to 5, said reaction being conducted at a temperature of about 365 C. in a conned reaction zone consisting essentially of at least one tube packed with an inert material having a ratio oi area to gas contacting surface to volume of free space in the range of about 5 to 15 square centimeters per cubic centimeter, while maintaining the sum of the partial pressures of hydrocarbon and oxygen of said air in the range of about 45 to 65 pounds per square inch.

7. A process for producing propylene oxide by direct oxidation which comprises reacting propylene with a molecular oxygen containing gas admixed in a molar ratio of propylene to oxygen content of about 5, said reaction being conducted at a temperature of about 365 C. in a confined reaction zone consisting essentially of at least one tube packed with an inert material having a ratio of area o1' gas contacting surface to volume cubic centimeter. while maintaining thesum of the partial pressures of hydrocarbon and oxygen at about 45 .pounds per square inch.

8. A process for producing propylene oxide by direct oxidation which comprises reacting a hydrocarbon mixture oi' about 15 per cent propane and 85 per cent propylene with a molecular oxygen containing gas when admixed in a molar ratio of hydrocarbon to oxygen content within the range of about 4 to 5. said reaction being conducted at a temperature within the range of about 365? C. toabout 385 C. in a confined reaction zone consisting essentially of at least one tube packed with an inert material having a ratio of area of gas contacting surface to volume of'iree space is about 8 square centimeters per cubic centimeter. while maintaining the sum o! the partial pressures of hydrocarbon and oxygen at about 60 pounds per square inch.

9. A process for producing propylene oxide by direct oxidation which comprises reacting propane witha molecular oxygen containing gas a,sao,5oo A admixed in a molar ratio of propane to oxygen content within the range of about 3'to 4. said reaction being conducted at a temperature within the range of about 365 C. to about 400 C.V in aconned reaction zone consisting essentially of at least lone tube packed with an inert material having a ratio of area of gas contacting surface to volume oi'iree space is about 8 square centimeters per cubic centimeter, while maintaining the sum of the .partialpressures of hydrocarbon and oxygen at about 60 pounds per square inch. i

GERHARD A; coox.

REFERENCES CITED y The following references are of record in the me of this patent:

UNITED STATES APl'rrEu'rs 2 Number Name Date 1,995,991 l Lenher Mar. 26, 1935 2,367,169 Gardner Jan. 9. 1945 

1. A PROCESS FOR PRODUCING PROPYLENE OXIDE BY DIRECT OXIDATION WHICH COMPRISES REACTING, AT A TEMPERATURE WITHIN THE RANGE OF ABOUT 275*C. TO ABOUT 700*C., HYDROCARBON GAS SELECTED FROM THE GROUP CONSISTING OF PROPANE AND PROPYLENE WITH A GAS CONTAINING MOLECULAR OXYGEN IN AMOUNT SUFFICIENT TO GIVE IN THE TOTAL GAS MIXTURE A MOLAR RATIO OF HYDORCARBON TO OXYGEN OF AT LEAST 2 AND NOT MORE THAN 20, WHILE CONFINING THE GASES IN A REACTION ZONE HAVING A RATIO OF AREA OF GAS CON- 